It is desirable for an amplitude modulation detector to produce an output whose D.C. voltage component is always constant. This simplifies the design of the audio frequency amplifier (AF amplifier), and also eliminates noise arising at the time of switching frequency bands or switching to or from other sources. Such sources can be, for instance, a frequency modulation detector or tape player combined with the amplitude modulation detector in a set. The noise is caused by a sudden change of the D.C. voltage component of the input signal to the AF amplifier.
Conventional amplitude modulation (AM) radio receivers have needed AF amplifiers having wide level response to the input signal from the amplitude modulation detectors. Moreover, multiband type conventional AM receivers or combination sets with an AM receiver section and other signal source section, such as a tape recorder, have used decoupling capacitors connected before the AF amplifiers.
However, the decoupling capacitors are difficult to fabricate when using integrated circuits (IC circuits).